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Can mDNS be used to resolve to a link-local address?

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0

In an earlier discussion on the topic of IPv6 Link-local addresses I saw a comment which stated:

Some would argue that using addresses instead of host names should be discouraged, for any type of addresses. Using mDNS to map to link-local addresses works fine.

The context of the discussion was that you need to specify an interface when using a link-local address.

I found the above comment surprising because I didn't expect mDNS to supply link information, thus I didn't expect it to "work fine". I constructed a simple test (below) to see if the information supplied by getaddrinfo included a link and I found the following:

• Passing an address and link (eg: fe80::a:a:a:a%wlp3s0) to getaddrinfo resulted in a sockaddr_in6 with sin6_scope_id set to 3 (the index of my wlan card).




• Passing a hostname only resolvable by mDNS (which resolves to the same link-local address) resulted in a sockaddr_in6 with sin6_scope_id set to 0, ie no link specified




• Further I've confirmed that programs including SSH which rely on getaddrinfo to specify a link will fail when mDNS resolves a link-local address. But they will succeed if the IP address and link are specified explicitly.

---

Was the above comment wrong, or have I made a mistake in the way I test this? Can mDNS resolve to link-local address and specify the link?

For reference I'm testing on Debian Buster Linux version 4.19.0-5-amd647

My nsswitch.conf contains

hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname

---

Testcode.c

#include <stdio.h>

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <arpa/inet.h>

#include <errno.h>

#include <stdlib.h>

#include <unistd.h>

#include <netdb.h>





int main(int arg_count, char ** args)

{

    struct addrinfo hints, *servinfo, *p;



    for (int i=1; i<arg_count; i++)

    {

        memset(&hints, 0, sizeof hints);

        hints.ai_family = AF_INET6; // to force IPv6

        hints.ai_socktype = SOCK_STREAM;

        printf("Checking %sn", args[i]);

        if (getaddrinfo(args[i], "https", &hints, &servinfo)) {

            perror("getaddrinfo");

            continue;

        }



        for(p = servinfo; p != NULL; p = p->ai_next) {

            struct sockaddr_in6 * address = ((struct sockaddr_in6 *)p->ai_addr);

            printf("family %d scope %dn", address->sin6_family, address->sin6_scope_id);

        }

        freeaddrinfo(servinfo);

    }



    return 0;

}

c ipv6

share|improve this question

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

add a comment | 

0

In an earlier discussion on the topic of IPv6 Link-local addresses I saw a comment which stated:

Some would argue that using addresses instead of host names should be discouraged, for any type of addresses. Using mDNS to map to link-local addresses works fine.

The context of the discussion was that you need to specify an interface when using a link-local address.

I found the above comment surprising because I didn't expect mDNS to supply link information, thus I didn't expect it to "work fine". I constructed a simple test (below) to see if the information supplied by getaddrinfo included a link and I found the following:

• Passing an address and link (eg: fe80::a:a:a:a%wlp3s0) to getaddrinfo resulted in a sockaddr_in6 with sin6_scope_id set to 3 (the index of my wlan card).




• Passing a hostname only resolvable by mDNS (which resolves to the same link-local address) resulted in a sockaddr_in6 with sin6_scope_id set to 0, ie no link specified




• Further I've confirmed that programs including SSH which rely on getaddrinfo to specify a link will fail when mDNS resolves a link-local address. But they will succeed if the IP address and link are specified explicitly.

---

Was the above comment wrong, or have I made a mistake in the way I test this? Can mDNS resolve to link-local address and specify the link?

For reference I'm testing on Debian Buster Linux version 4.19.0-5-amd647

My nsswitch.conf contains

hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname

---

Testcode.c

#include <stdio.h>

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <arpa/inet.h>

#include <errno.h>

#include <stdlib.h>

#include <unistd.h>

#include <netdb.h>





int main(int arg_count, char ** args)

{

    struct addrinfo hints, *servinfo, *p;



    for (int i=1; i<arg_count; i++)

    {

        memset(&hints, 0, sizeof hints);

        hints.ai_family = AF_INET6; // to force IPv6

        hints.ai_socktype = SOCK_STREAM;

        printf("Checking %sn", args[i]);

        if (getaddrinfo(args[i], "https", &hints, &servinfo)) {

            perror("getaddrinfo");

            continue;

        }



        for(p = servinfo; p != NULL; p = p->ai_next) {

            struct sockaddr_in6 * address = ((struct sockaddr_in6 *)p->ai_addr);

            printf("family %d scope %dn", address->sin6_family, address->sin6_scope_id);

        }

        freeaddrinfo(servinfo);

    }



    return 0;

}

c ipv6

share|improve this question

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

add a comment | 

In an earlier discussion on the topic of IPv6 Link-local addresses I saw a comment which stated:

Some would argue that using addresses instead of host names should be discouraged, for any type of addresses. Using mDNS to map to link-local addresses works fine.

The context of the discussion was that you need to specify an interface when using a link-local address.

I found the above comment surprising because I didn't expect mDNS to supply link information, thus I didn't expect it to "work fine". I constructed a simple test (below) to see if the information supplied by getaddrinfo included a link and I found the following:

• Passing an address and link (eg: fe80::a:a:a:a%wlp3s0) to getaddrinfo resulted in a sockaddr_in6 with sin6_scope_id set to 3 (the index of my wlan card).




• Passing a hostname only resolvable by mDNS (which resolves to the same link-local address) resulted in a sockaddr_in6 with sin6_scope_id set to 0, ie no link specified




• Further I've confirmed that programs including SSH which rely on getaddrinfo to specify a link will fail when mDNS resolves a link-local address. But they will succeed if the IP address and link are specified explicitly.

---

Was the above comment wrong, or have I made a mistake in the way I test this? Can mDNS resolve to link-local address and specify the link?

For reference I'm testing on Debian Buster Linux version 4.19.0-5-amd647

My nsswitch.conf contains

hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname

---

Testcode.c

#include <stdio.h>

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <arpa/inet.h>

#include <errno.h>

#include <stdlib.h>

#include <unistd.h>

#include <netdb.h>





int main(int arg_count, char ** args)

{

    struct addrinfo hints, *servinfo, *p;



    for (int i=1; i<arg_count; i++)

    {

        memset(&hints, 0, sizeof hints);

        hints.ai_family = AF_INET6; // to force IPv6

        hints.ai_socktype = SOCK_STREAM;

        printf("Checking %sn", args[i]);

        if (getaddrinfo(args[i], "https", &hints, &servinfo)) {

            perror("getaddrinfo");

            continue;

        }



        for(p = servinfo; p != NULL; p = p->ai_next) {

            struct sockaddr_in6 * address = ((struct sockaddr_in6 *)p->ai_addr);

            printf("family %d scope %dn", address->sin6_family, address->sin6_scope_id);

        }

        freeaddrinfo(servinfo);

    }



    return 0;

}

c ipv6

share|improve this question

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname

#include <stdio.h>

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <arpa/inet.h>

#include <errno.h>

#include <stdlib.h>

#include <unistd.h>

#include <netdb.h>





int main(int arg_count, char ** args)

{

    struct addrinfo hints, *servinfo, *p;



    for (int i=1; i<arg_count; i++)

    {

        memset(&hints, 0, sizeof hints);

        hints.ai_family = AF_INET6; // to force IPv6

        hints.ai_socktype = SOCK_STREAM;

        printf("Checking %sn", args[i]);

        if (getaddrinfo(args[i], "https", &hints, &servinfo)) {

            perror("getaddrinfo");

            continue;

        }



        for(p = servinfo; p != NULL; p = p->ai_next) {

            struct sockaddr_in6 * address = ((struct sockaddr_in6 *)p->ai_addr);

            printf("family %d scope %dn", address->sin6_family, address->sin6_scope_id);

        }

        freeaddrinfo(servinfo);

    }



    return 0;

}

share|improve this question

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

share|improve this question

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

edited 54 mins ago

JL2210

1637

edited 54 mins ago

JL2210

1637

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427

asked 2 hours ago

Philip CoulingPhilip Couling

3,61011427